1. Field of the Invention
This invention relates to injection molding presses and more particularly to rotary injection molding presses which inject fluid plastic into a part forming mold.
2. Description of the Prior Art
In injection molding, molds are provided which have cavities to receive plastic heated to its fluid state. The plastic, as it cools, hardens and conforms to the cavity configuration resulting in the desired shaped plastic part. Components such as electrical wiring or the like are often positioned within a cavity so as to become embedded within the finished plastic part.
One known injection molding device to produce the aforesaid plastic parts is a rotary injection molding press. The press has a frame supporting a rotatable table which mounts one or more mold blocks. The mold blocks, designed and constructed for each particular configuration of plastic part, have a bottom secured to the table and a top. The top is hinged to the bottom to permit the mold block to open and to close to define the plastic receiving mold cavity. Typically, the mold cavity is fashioned in the mold block bottom with the top having a female opening to receive and admit fluid plastic into the cavity.
The rotary injection molding press also includes an injection mechanism which is fed with solid plastic, usually in pellet form, and which includes heating elements to heat the plastic to a fluid state. A piston-like plunger element housed within the injection mechanism is driven to force the fluid plastic under pressure through an injector nozzle into the cavity of a properly positioned mold block. Most of the aforesaid presses have a number of mold blocks secured to the platform so that plastic parts can be fashioned in a continuous manner.
In a typical prior art rotary injection molding press, the platform can be said to sequentially position one or more mold blocks at specific stations on the press. At the inject station, a closed mold block is positioned for engagement by the plunger and is injected with fluid plastic. After the mold block has been injected with fluid plastic, the platform is moved by indexing means to transport the mold block to a cooling station wherein the fluid plastic within the mold cavity is allowed to cool and harden to form the desired plastic part. Next, the platform moves to index the mold block to an eject station where the molded plastic part is removed from the mold block. Subsequent to the removal of the molded part from the mold block, the platform indexes to transport the mold back to and through a prep station where the mold block is again made ready to receive plastic as by position of electrical wiring or the like therein, and the mold block is closed.
One of the drawbacks of the injection of rotary molding press described above is that should a mold block be inadvertently left open as the platform moves to index the mold block to the inject station, the open mold block as it proceeds to the inject station can engage the plunger damaging either the mold block or the plunger or both. Furthermore, plastic injected toward an open mold block is not confined to the mold cavity. This results in the plastic spreading over the mold block, table and surrounding structure. The foregoing is costly since labor is required for clean-up and during the shut-down of the machine the desired parts are not being produced.
Another drawback focuses upon the inadvertent failure to open the mold block prior to the eject station of the press. In many presses hydraulic rams extend from beneath the platform, through the mold block bottom to eject the molded piece from the mold block. Accordingly, should the mold block top be left closed actuation of the hydraulic ram can damage either the piece, the mold block or the ram itself. Again, the foregoing is costly not only due to the repairs required and the machine down time but also due to the damaged piece which must be scrapped.
A further drawback is that in many injection molding presses, the plunger and its movement are exposed. An inattentive operator is susceptible to injury should a portion of his body be placed in the path of the plunger. Furthermore, the heating elements, while somewhat shielded within the plunger mechanism, do produce elevated temperatures sufficient to burn the operator. Protective shrouds have been provided. However, to view the operation of the plunger many of the operators often remove and discard the protective shroud exposing themselves to risk of personal injury.